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WebAssembly Text Format (WAT)

WA offers a notation which can be used to write human-readable code, compiled to WASM. This so called WebAssembly Text Format looks a little similar to Assembler, which you might have seen before.

Installing needed tools​

For compiling your WAT code to WASM, I recommend using wat2wasm.

Installing with NPM: npm install -g wat-wasm Installing it with brew: brew install wabt

Alternatively, check out: https://github.com/webassembly/wabt

Writing a small program with WAT​

(module
  (func (export "sum")
    (param $a i32)
    (param $b i32)
    (result i32)
    local.get $a 
    local.get $b 

    i32.add 
  )
)

Compiling it:

wat2wasm add.wat -o add.wasm

Running it with node.js​

Now, we can load this file in Node.js and use the function: 

const fs = require('fs')
const bytes = fs.readFileSync(__dirname + '/add.wasm')

(async () => {
  const obj = await WebAssembly.instantiate(new Uint8Array(bytes))

  let value = obj.instance.exports.sum(7, 5)
  console.log(value)
})()

node index.js should log "12" now.

Running it in the browser​

All you have to do, is to write the following JavaScript code and use it in the browser, so embed it in your HTML or you the . 

fetch('add.wasm')
  .then((response) => response.arrayBuffer())
  .then((bytes) => WebAssembly.instantiate(bytes))
  .then((response) => {
    let result = response.instance.exports.sum(7, 5)
    console.log(result)
  })

Understanding the stack machine​

(module
  (func (param i32) (param i32) (result i32)
    local.get 0
    local.get 1
    i32.add 
  )
)

We can access the parameters via index or via name. via index would look like this: local.get 0 and local.get 1.

Getting them via name is shown in the first code example.

The add-function takes the top-2 elements, removes them from the stack and push the result on the stack.

Writing comments​

;; This is a single line comment 

(; Multiline
Comment ;)

Data types​

For now, there are integers and floats, defined by the size in memory: i32, i64, f32, f64